Polyesters typified by polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate excel in mechanical properties and chemical properties, and depending upon their properties, they are used in a wide variety of fields including fibers for clothes and industrial materials, films or sheets for packaging materials or magnetic tapes, bottles, which are hollow molded articles, casings of electric or electronic appliances, and other types of molded articles or components.
A representative polyester, namely, a polyester composed of an aromatic dicarboxylic acid component and an alkylene glycol component as major constituents, such as polyethylene terephthalate, is produced by first preparing bis(2-hydroxyethyl)-terephthalate (BHET) and an oligomer containing the same by an esterification reaction between terephthalic acid and ethylene glycol or a transesterification reaction between dimethyl terephthalate and ethylene glycol, and then subjecting the oligomer to melt-polycondensation in a vacuum at high temperatures in the presence of a polycondensation catalyst.
For use in production of biaxially stretched bottles made from polyester known as “plastic bottles”, a polyester having a higher molecular weight than polyesters used for fibers and films are needed so that the resulting bottles have enough strength. Accordingly, a polyester having a higher molecular weight which is obtained by solid-polycondensation of polyester obtained as melt-polycondensate is used.
As such a polycondensation catalyst for producing polyester, antimony trioxide has been heretofore widely used. Antimony trioxide is a catalyst which is inexpensive and is of excellent catalytic activity; however, it has some problems. For example, antimony metal is deposited while it is used in polycondensation of polyester raw materials, thereby making the resulting polyester darkened, or the resulting polyester is contaminated with the antimony metal deposited.
It is already known that in production of polyester, coloration of polyester obtained is prevented when an alkali such as sodium hydroxide or potassium hydroxide is present in the reaction system together with a catalyst (see patent literature 1). In the case of antimony trioxide catalyst, too, it is already known that color tone of polyester obtained can be improved by co-use of a predetermined amount of sodium oxide together with an iron oxide (see patent literature 2). However, because antimony trioxide is inherently poisonous, development of catalysts free of antimony has been awaited in recent years.
Under these circumstances, as a polycondensation catalyst for producing polyester by a transesterification reaction between dimethyl terephthalate and ethylene glycol, a glycol titanate (see patent literature 3) and a tetraalkoxy titanium (see patent literature 4) have been proposed, for example. In recent years, there has been proposed as a polycondensation catalyst a solid titanium compound which is obtained by hydrolyzing a titanium halide or a titanium alkoxide to prepare a hydroxide of titanium, and then dehydrating and drying the hydroxide by heating it at a temperature of from 30° C. to 350° C. (see patent literatures 5 and 6).
The titanium-based catalysts described above have high polymerization activity in many cases. However, coloration to yellow is seen in the resulting polyester obtained by using such titanium-based catalysts. Furthermore, the resulting polyester is liable to be colored due to thermal degradation during the stage of melt-molding. There is also a tendency that the resulting polyester is poor in transparency.
In order to solve the above-mentioned problems, a titanic acid catalyst having a particulate structure which comprises a particle of a solid base such as magnesium hydroxide and hydrotalcite having on the surface a coating formed of titanic acid (see patent literature 7).
Such a titanic acid catalyst having a particulate structure provides polyester superior in color tone and transparency at a polymerization activity equal to or higher than the antimony trioxide catalyst in production of polyester by melt-polycondensation of polyester raw materials. In particular, the titanic acid catalyst has a higher polymerization activity than the antimony trioxide catalyst from the point of view of polycondensation rate in melt-polycondensation of polyester raw materials.
However, when the polyester obtained by using the titanic acid catalyst is subjected to solid-polycondensation to obtain a polyester having a higher molecular weight, it has been found that the catalyst leaves much room for improvement in catalytic activity, in particular, in terms of a polycondensation rate as compared with the antimony trioxide catalyst.